transfer log reverse lookup possible to disable?

In the transfer logs (in logs/domain.com directory) I see that all website-visitors IP-addresses are reverse lookupped.

However in some situations (especially with spammers) the reverselookup results in a a host name I cannot trace back to an IP.

examples:
hn.kd.ny.adsl
unknown.altushost.com

Question: is there a way to disable reverse lookup on the domain transferlog files (on Unix plans)?

Or is there another way to get the IP of those clients?

Thanks,
Jan-Pieter

Could you provide us domain name in ticket.